Acoustic emissions are small sound waves caused by microseismic activity within any material. In steel bridges, such acoustic emissions are caused by fatigue crack growth, friction of crack surfaces, rubbing at connections, noise directly generated by traffic, and the like. The monitoring of these microseismic waves can be accomplished by one or more piezoelectric transducers which convert the mechanical waves to electrical signals. The detection, amplification, counting, filtering and analyzing of these signals constitute the present state of the art in acoustic emission technology. Subsequent data analysis generally falls into two categories including: (i) analysis for acoustic event rate, event count, and frequency characteristics; and, (ii) analysis for location of the source of the acoustic emission event based on the time of arrival of the same acoustic emission event at a number of different transducers.
Structural monitoring for containment structures, e.g., pressure vessels, has been a driving force behind development of much of the acoustic emission instrumentation currently available. Materials Evaluation, "Acoustic Emission Testing" Feb. 1988, presents a general review of developments in industrial applications of monitoring techniques. Bridge monitoring is generally discussed by Gong et al., "Acoustic Emission Monitoring of Steel Railroad Bridges", Materials Evaluation, July 1992, pp. 883-887, and by Carlyle et al., in 1993 reports to the U.S. Department of Transportation regarding acoustic emission monitoring of the I-205 Willamette River bridge and the I-10 Mississippi River bridge. Proper techniques for the inspection of a structure such as a bridge superstructure are important in making rational decisions regarding rehabilitation, repair or replacement. With the increased public awareness of the decaying national infrastructure, the search for improved nondestructive evaluation techniques is magnified. Despite all the work in the area of nondestructive evaluation techniques, present techniques suffer from deficiencies including, e.g., a certain level of subjectiveness.
It is an object of the present invention to provide a process of determining a rough or approximate location of an acoustic emission event using a single transducer.
Another object of the invention is to provide a process of determining the source location of an acoustic emission event using the vibration modes of the structure together with the generated Lamb waves.
Still another object of the present invention is to provide a process for acoustic emission event monitoring with reduced subjective nature.